public class Test {
/*
电话号码的字母组合
class Solution {
    String[] digit = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    List<String> ret ;
    StringBuffer path = new StringBuffer();
    public List<String> letterCombinations(String digits) {
        ret = new ArrayList<>();
        if(digits.length() == 0) {
            return ret;
        }
        dfs(digits,0);
        return ret;
    }
    void dfs(String digits , int pos) {
        if(pos == digits.length()) {
            ret.add(path.toString());
            return;
        }
        String cur = digit[digits.charAt(pos) - '0'];
        for(int i = 0;i < cur.length();i++) {
            char ch = cur.charAt(i);
            path.append(ch);
            dfs(digits,pos+1);
            path.deleteCharAt(path.length()-1);
        }
    }
}

缺失的第一个正数
class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            while (nums[i] >= 1 && nums[i] <= n && nums[i] != nums[nums[i] - 1]) {
                // swap
                int temp = nums[nums[i] - 1];
                nums[nums[i] - 1] = nums[i];
                nums[i] = temp;

            }
        }
        for (int i = 0; i < n; i++) {
            if ((i + 1) != nums[i]) {
                return i + 1;
            }
        }
        return n + 1;
    }
}
 */


}
